In Long Term Evolution (LTE) cellular communications networks, link adaptation is based on Adaptive Modulation and Coding (AMC). In general, each base station (i.e., enhanced Node B or eNodeB) adaptively controls a Modulation and Coding Scheme (MCS) for downlink transmissions to each mobile terminal (i.e., User Equipment or UE) served by the base station. Currently, LTE cellular communications networks support three modulation types, namely, Quadrature Phase Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (QAM), and 64 QAM and a wide range of code rates that together provide numerous MCSs. More specifically, each mobile terminal periodically sends Channel State Information (CSI) reports to the base station and, based on the CSI reports from the mobile terminal, the base station adaptively controls the MCS for downlink transmissions to the mobile terminal.
An additional input to link adaptation is Hybrid Automatic Repeat Request (HARQ) acknowledgements (i.e., HARQ ACKs and HARQ NACKs) received from the mobile terminal in response to downlink transmissions to the mobile terminal. More specifically, as part of the HARQ functionality of the base station, the base station receives HARQ acknowledgements from each of the mobile terminals in response to downlink transmissions to the mobile terminal. The base station adaptively controls the MCS for downlink transmissions to each mobile terminal based on the CSI reports from the mobile terminal and the HARQ acknowledgements from the mobile terminal.
However, the inventors have found that when using the conventional adaptive MCS selection techniques described above, a Block Error Rate (BLER) for downlink transmissions to the mobile terminal may vary between subframes. For example, the inventors have found that when using the conventional adaptive MCS selection techniques described above, the BLER for some subframes can be as high as 50% whereas the BLER for other subframes can be as low as 0%. As a result, downlink throughput suffers. As such, there is a need for systems and methods for link adaptation that provide improved downlink throughput.